While we wait for the packs to be delivered in EU, I was wondering if the non duplicate legendary rule will apply in the golden classic pack, since blizzard handles it as something totally separate from the classic pack

I would bet it doesnt contain any duplicates given the case you still lack some golden legendaries.

The no-dupe rule doesn’t care about golden cards, so if you get a (golden) legendary it will be one you don’t already have. If you have all classic legendaries then you’ll obviously get a duplicate, albeit golden.